Electrodeionization (EDI) is best known as a desalting process for dilute aqueous streams. Its commercial application has been limited to the production of ultra-pure water mainly for semiconductor and pharmaceutical industries. EDI technology based on a fixed resin-wafer has been disclosed in U.S. Pat. No. 6,495,014, the entire disclosure of which is incorporated by reference. Further disclosures of related EDI technology are in U.S. patent application Ser. No. 10/213,721 filed Aug. 6, 2002 entitled “ELECTRODEIONIZATION METHOD” and U.S. patent application Ser. No. 10/702,798 filed Nov. 5, 2003 entitled “IMMOBILIZED BIOCATALYTIC ENZYMES IN ELECTRODEIONIZATION (EDI)”, the entire disclosures of which are incorporated by reference. This technology was originally developed for desalting industrial dextrose streams but its use can be extended to the application of EDI in the fields of chemical production, separation, and purification.
Organic esters, such as ethyl lactate, are attractive substitutes for many traditional solvents that are generally considered to be toxic. For this description, ethyl lactate is used as an example of a class of organic acid esters that are derived from the reaction of small alcohols such as methyl, ethyl, propyl, butyl, etc. with organic acids such as acetic, lactic, propionic, 3-hydroxy-propionic, butyric, etc. Ethyl lactate is a biodegradable chemical that has equivalent or superior solvent properties compared to many petroleum-based solvents. It is manufactured by esterification, i.e., reaction of ethanol and lactic acid. Although lactic acid is produced by fermentation, the acid is neutralized to the lactate salt to prevent media acidification and inhibition of biocatalytic activity. Therefore, the lactate salt must be converted back to lactic acid and the acid must be recovered and purified before esterification. The salt conversion and subsequent acid recovery can be the highest cost in the entire ester process, and is the greatest economic barrier to increased ethyl lactate production and utilization.
Various methods have been implemented or proposed to convert the salt to lactic acid. The utility of these methods can be affected by the type of base that is used to neutralize the lactic acid. The conventional approach uses hydrated lime for neutralization to form calcium lactate salt. Following fermentation the broth is acidified with sulfuric acid, which produces calcium sulfate and lactic acid. Calcium sulfate, which is only slightly soluble, precipitates during the acidification step and is removed from the broth by filtration. Although simple, this approach requires the addition of acid and produces nearly one pound of waste calcium sulfate for every pound of lactic acid that is produced.
Another approach uses sodium hydroxide or bicarbonate as the base for neutralization and a double electrodialysis process to concentrate the sodium lactate salt by desalting electrodialysis (DSED) and to split the salt into sodium hydroxide and lactic acid by water-splitting electrodialysis (WSED); the sodium hydroxide can then be recycled back to the fermentor. This approach is somewhat more economical than the conventional process because the DSED and WSED processes purify the lactate, as well as convert and separate it from the broth, and produce much less chemical waste. The DSED and WSED systems and membranes, however, represent significant capital and operating costs.
Yet another approach, referred to as direct esterification, uses ammonium hydroxide to neutralize the acid in the fermentor, DSED to concentrate and purify the ammonium lactate salt, and a pervaporation-assisted reactor to “thermally crack” ammonia gas from the ammonium lactate and remove it from the broth and to simultaneously carry out the esterification reaction. The primary difficulty with this approach is that ammonia and ethyl lactate can react irreversibly to produce lactamide that has no significant value and substantially reduces the ethyl lactate yield.